1. Field of the Invention
This invention relates to a wide-angle objective for endoscopes, in particular for video endoscopes having a semiconductor image transducer and for endoscopes having a fibre image conductor, the wide angle objective being of the kind including a meniscus located at the distal and object side with its concave surface facing towards the object, another structural optical component located at the image side, and a diaphragm possibly associated with the optical system.
2. Description of the Prior Art
Objectives of this kind are required to have a short structural length, with a small diameter, but in the endoscope optical systems hitherto known to the applicant, this cannot be accomplished without difficulty or only by a commensurate degree of complexity, if a large image or object field angle and satisfactory correction of the optical system are to be obtained.
Furthermore, there are specific limits for the structural length since minimum values apply to the central thicknesses, the marginal thicknesses as well as to the vertex spacings of the lenses, so that the structural length of the objectives cannot be reduced at will.
If, for example, the objective comprises two achromatic lenses, it is impossible to obtain perfect correction of the image field curvature since the negative refractive capacities needed for this purpose are missing from the optical system as a whole.
The result is that there is considerable attenuation of the definition in the marginal portion. For this reason, optical systems of this kind are commonly applicable only if the required object field angle amounts to at the most 60.degree., unless one is prepared to be involved in the high cost of limiting the image defects by the use of marginal vignetting.
Now, in the known flexible video endoscopes having an image transducer in the form of a chip, the rigid, distal end portion extends distally beyond the structural length of the objective because of the image transducer, the electronic system and the like. Furthermore, the distal end of the fibre cord of flexible endoscopes having a fibre image conductor is rigidly bonded into a metal sleeve which adds to the structural length of the objective. Thus, it will be appreciated with such endoscopes that the rigid, distal end portions of the endoscopes should be as short as possible and that the shortness of the objective is of decisive importance.